Heretofore, when articles have been injection molded between a core element and a pair of mold halves enclosing said core element, but spaced therefrom, it has been necessary to provide a tapering of the core element such that its peripheral surface assumes at least a slight frusto-conical configuration with the smaller end of the core being that end which is first moved into the cavity defined by the two mold halves. Such tapering of the core has always been considered necessary in injection molding in order to enable the core to be withdrawn from the hollow article after the latter has been formed and the plastic material of which it has been molded, has cooled sufficiently for handling. This is because, with the cooling of the plastic material, such shrinkage occurs that, if no tapering of the core is provided, the core becomes so tightly gripped by the material that any effort forceably to pull the core out of the thus-molded article, results in damage to the latter. Conventional industry practice, therefore, has always dic-tated providing at least some tapering of the core.
While such tapering has not prevented the fabrication of many types of hollow plastic articles, where tapering of the inside wall of the article has not interfered with the use or appearance of the article, the tapering requirement has prevented injection molding of those hollow walled articles, where both the inside, as well as outside walls, have been required to be perfectly cylindrical in configuration. While certain of such cylindrical articles could be satisfactorily extruded, other articles fabricated of certain types of plastic material, have not been able to be so extruded.
Prior to the present invention, it has simply not been possible to injection mold hollow perfectly cylindrical articles in one piece. What has been done, therefore, has been to mold separately two halves of the article, by means of mating molds which can be simply pulled apart. The two separately molded halves have then been joined together along their edges by means of an adhesive, or some type of thermoplastic jointure. Articles made in this latter manner, however, will ordinarily be found to have visible seams which may not be desired in certain applications, and, in most instances, such articles may require some further machining to smooth over the seamed junctures. Such procedure, moreover, is cumbersome, time-consuming and expensive.
There has thus been a great need in the plastics industry for the development of a technique and apparatus whereby straight-walled hollow articles may be injection molded through the use of an inserted and withdrawn core: but prior to the present invention, no such technique has been developed nor has any apparatus been available for accomplishing this objective.